In the course of manufacturing paper and similar products, including paperboard and the like, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. A number of inorganic materials have long been known to be effective for these purposes, such as titanium dioxide, which can be incorporated into the paper in the form of anatase or rutile. Titanium dioxide, however, is among the most expensive of known fillers. Accordingly, in recent years, considerable efforts have been made to develop satisfactory replacements for titanium dioxide.
Among the materials which have found acceptance as paper fillers are substantially anhydrous kaolin clays. Materials of this type are generally prepared by partially or fully calcining a crude kaolin clay, which may have been initially subjected to beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product.
An "hydrous" kaolin generally means that the kaolin has not been subjected to calcination, or more specifically, it has not been heated to temperatures above about 450.degree. C.
The term "anhydrous" generally refers to a kaolin clay which has been subjected to calcination, which means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups thereby destroying the crystalline structure of the kaolinite. A "fully calcined" kaolin refers to kaolins which have been heated above the 980.degree. C. exotherm, as opposed to having been rendered merely a "metakaolin". Reference may be had in the foregoing connection to Proctor U.S. Pat. No. 3,014,836 and to Fanselow et al U.S. Pat. No. 3,586,823, which disclosures are representative of portions of the prior art pertinent to fully calcined kaolins.
"Calcined" kaolin refers to an anhydrous kaolin clay such as the ALPHATEX.RTM. product of the assignee of the present invention. This product is a substantially anhydrous white kaolin clay pigment, which has unusual efficacy as a filler in paper sheets and similar paper products. This pigment also has application as a coating pigment for paper and as a pigment in paints and other filled systems. It generally consists of aggregates of anhydrous kaolin clay particles, and exhibits exceptionally high light-scatter and opacifying characteristics when incorporated as a filler in paper.
The ALPHATEX.RTM. product is further described in U.S. Pat. No. 4,381,948 to A. D. McConnell et al. and in U.S. Pat. No. 5,261,956 to W. H. Dunaway et al., both of which are assigned to the assignee of the present invention.
Calcined kaolin clay products, such as the ALPHATEX.RTM. product are often handled, shipped, and/or utilized (e.g. when coating papers) as aqueous slurries, i.e. the calcined kaolin clay is slurried in water to form a slurry, e.g. of approximately 40% to 60% solids content, more or less. The more viscous the product, generally, the more difficult it is to handle and to use. In the past, this has proven to be a problem, particularly where the solids content of the slurry exceeds about 50%, by weight.
For hydrous clays, the general problem of reducing clay viscosity has commonly involved chemical control by additives or elimination of a species tending to cause viscosity problems. Viscosity control in a calcined clay slurry often involves some type of milling which may be wet or dry, or comminution of the calcined material.
Wet milling involves exposing a fluid suspension of slurry of calcined kaolin clay to a grinding media, whereby incremental additions of the calcined kaolin clay are added and the milling is continued until the viscosity of the slurry is reduced. This process may have energy inputs of 200 hp-hr/ton, and is very difficult from an engineering viewpoint, requiring very complex and sophisticated controls.
Dry grinding of calcined kaolins is taught in the present assignee's U.S. Pat. Nos. 4,593,860 and 4,693,427, wherein the calcined kaolin clay is frictionally worked or milled by a media in the mill which is at least +5 mesh to decrease its clay-water viscosity and adhesiveness. The work inputs in the prior art may be from about 5 to about 40 hp-hr/ton of dry clay. The process of the prior art improves the handling characteristics for the treated clay with respect to bulk material handling systems, but an improved coating pigment would still be desirable.
U.S. Pat. No. 4,868,228 discloses the use of fully neutralized acid acrylic polymers as wet grinding aids for aqueous suspensions of coarse mineral materials, such as calcium carbonates. Monovalent and polyvalent neutralizing agents are used to prepare these wet grinding aids. This reference does not disclose or suggest the use of dry mill grinding aids for calcined kaolin. It also does not disclose or suggest the use of ammonium polyacrylates as dry grinding aids.
In common paper coating applications, the pigment properties often sought include high gloss, good printability, good opacity, and high brightness. Where calcined kaolins are used, manufacturers generally prefer to utilize a paper coating composition (the "coating color") which desirably incorporates from 15 to 20 parts per hundred by weight of the calcined kaolin in place of the more expensive pigments such as titanium dioxide. The coating color should include about 60 to 63% by weight of the total solids in order to be most effective. With the prior art calcined pigments, all of these requirements could not be readily achieved. In particular, in order to achieve calcined kaolin having 60 to 63% solids coating color, it is generally required by a customer to be able to use or handle the kaolin pigment at its equivalent of 60% solids, which is the percent weight of the solids (i.e. clays) in the liquid of the slurry. That is, the overall requirement is that the kaolin pigment have good rheological properties as at least a 60% solids aqueous slurry and also provide fully acceptable opacity, gloss, brightness, and printability when coated. The pigments of the prior art, while capable of being formed into such high solids slurries, tend to have very high dilitancy, and thus, poor rheological properties. In an effort to allow the prior art calcined pigments to be used, lower solids can be employed, but this tends to leave excess water in the coating color, with resultant running during coating.
A further property of the prior art calcined pigments that has diminished their value as coating pigments arises from the very porous, highly aggregated nature of their component structures. While these properties foster high opacity, i.e. good light scattering, they have a detrimental effect on gloss when the pigment is used in coating.
In order to overcome some of the shortcomings of the prior art calcined pigments, a method for improving the rheology of a calcined kaolin clay product is taught in the aforementioned U.S. Pat. No. 5,261,956. The resultant product is commercially available as DELTATEX.RTM. from ECC International Inc., which is the same assignee as that of the ALPHATEX.RTM. product and of the present invention. The DELTATEX.RTM. product is a calcined kaolin coating pigment, which displays good rheology in very high solids aqueous slurries, such as 59%, thereby facilitating its use in coating and in the preparation of coating colors, and which provides high gloss, high brightness, and good opacity and printability characteristics in the paper upon which it is applied.
It is known in the art to add ammonium polyacrylate as a dispersant to neutralize the charges on the surface of the clay particles during blunging. Proper dispersion is necessary during wet processing in order to separate the clay stacks so as to optimize liberation of the impurities trapped between the clay particles in the kaolin clay slurry. This is also necessary in order to achieve a rheology suitable for further processing, i.e. pumping or storing the kaolin clay slurry in tanks.
Being that the DELTATEX.RTM. product is a very desirable commercial commodity for use as a pigment for paper and paperboard coating applications, it is most advantageous to be able to increase the production rate of this product. One of the drawbacks to increasing production rates is believed to exist in the dry grinding process through the media mill. It is hypothesized that the static electricity in the dry media mill due to the dry, fine particles of the calcined clay powder oftentimes lessens the degree of flowability of the powder through the mill, thereby causing clogging and/or plugging of the mill, or causing the clay to stick to the media balls used in the mill and/or to the sides of the mill, resulting in downtime of the mill.